The present invention relates to protective relays in general, and more particularly, to an electronic relay for detecting a reverse power condition of a three-phase alternating current generator for the protection against generator motoring.
In a power system network which may be powered by at least one conventional turbine-generator set, for example, when the turbine drive of the generator cannot meet all of the losses of the set, the deficiency in real power is absorbed from the power system network into the generator, commonly referred to as a reverse power condition, causing the potential of generator motoring. Relays for protecting against generator motoring are normally designed to be extremely sensitive to respond to the first increment of reverse power so as to prevent the motoring condition. Reverse power, i.e. the real power being absorbed from the power system network into the generator, as small as 0.03% of the nameplate rating of the generator may be enough to cause motoring. Most, if not all relays, encounter difficulties in detecting reverse power of this magnitude and still carry continuous load current in the forward direction under normal conditions. For more information on protection against generator motoring, reference is hereby made to the Westinghouse publication "Applied Protective Relaying", 1979, pp. 6-12 and 6-13.
A typical anti-motoring or reverse power relay of the electro-mechanical type is the Westinghouse CRN-1 which is described in the Westinghouse publication I.L. 41-251.2M, July, 1985. The CRN-1 relay comprises a cylinder directional unit which detects the reverse power condition and controls the timing of an induction-disc voltage-timing unit. This relay has a sensitivity of 0.02 amp at rated nominal current of 5 amperes.
Recently, the sensitivity requirement for anti-motoring protection has been increased beyond that heretofore achieved; while, the requirement of withstanding current levels which may exceed fifteen times the rating (5 amperes) of a measuring current transformer, i.e. on the order of 75 amps, for example, was maintained. Applicants have met this challenge with an all-electronic protective relay embodiment purportedly capable of measuring line current as low as 0.02% of the normal current transformer's rating for a wide range of phase-to-phase voltage values and still withstand the aforementioned high current levels it may encounter in the forward direction.